In coal mines and at other mining, industrial and building sites, electrical cables are subject to repeated environmental and mechanical stress which damage and degrade the cables by opening cracks and pin holes through insulation. These cracks and holes may eventually expose line conductors which present a life threatening hazard to miners and workers.
Of the cracks and openings in cables, pin holes are the most dangerous because a person is completely unaware of the opening's existence. Contact with a conductor through a pin hole often has a lethal result. Current or amperage (amps) is the physical phenomenon that damages a human and is therefore the most important parameter to monitor and control. At the voltage level, driving most mining, construction and industrial machinery, an amperage of only ten milliamps (0.010 Amperes) at a frequency of 60 Hz is sufficient to be lethal.
The amount of amperage that would flow through the body after contacting an exposed electrical cable is governed by Ohm's law, i.e.; voltage=amperage times resistance or amperage=voltage/resistance. The significance of this equation and its relation to hazardous amperage in operations such as coal mining is evident from the following example. A high enough internal resistance to voltages which puncture the skin and negate the effect of skin resistance is selected to be about ten thousand (10,000) ohms, which is high for any human but is selected to provide a safe upper limit. Considering a coal miner's activities as an example, as the miner is repositioning a 277 volt trailing cable of a shuttle car, the miner grabs the cable in a location where a conductor therein is exposed by a pin hole. From ohm's law, the amperage flowing through the coal miner is 277 volts divided by 10,000 which equals 0.0277 amperes or 27 milliamps, an amperage exceeding 10 milliamps which is more than enough amperage to present a lethal threat to the coal miner.
Research has shown consistently that a high percentage of electrical coal mining accidents, including some fatalities, are the direct result of a miner contacting an exposed electrical cable. There are many types of faulty conditions for which protective relaying is provided. However, to date, no reliable protection has been provided effectively in coal mines to minimize this cause of injury and fatality.